A radio frequency (RF) power amplifier (PA) is used to amplify an RF signal by converting direct current (DC) power to RF power. RF PAs are commonly found in wireless communication devices for driving the antenna(s) of a transmitter. The power consumption of an RF PA is often an important figure of merit because these wireless communication devices, such as mobile user equipment (UE) in cellular networks, are often battery operated. However, even in non-battery operated wireless communication devices, such as cellular base stations (BSs), the power consumption of an RF PA may still be an important consideration given that a large majority of operating expenditures associated with these devices is often energy.
Two metrics commonly used to measure the efficiency of a power amplifier include drain/collector efficiency η and power added efficiency (PAE). Drain/collector efficiency η provides a measure of how much of the DC input power to a PA is converted to RF output power and is given by the ratio of the RF output power of the PA to the DC input power of the PA. PAE primarily differs from drain/collector efficiency η in that it takes into further consideration the power of the input RF signal to the PA. PAE is given by:
  PAE  =                              P          out                -                  P                      i            ⁢                                                  ⁢            n                                      P                  D          ⁢                                          ⁢          C                      ×    100    ⁢    %  where Pout is the RF output power of the PA, PDC is the DC input power of the PA, and Pin is the power of the RF input signal to the PA.
Traditionally, linear PAs are biased so that there is sufficient DC input power to supply for peak RF output power conditions. Peak RF output power conditions generally occur when the RF input signal to the PA is at a maximum. However, when the PA is “backed-off” from the peak RF output power conditions, the excess DC input power must be dissipated by the PA because it is not being transformed into useful RF output power. Thus, PAs are generally most efficient during peak RF output power conditions.
FIG. 1A illustrates a plot 100 of PAE versus RF output power for an exemplary PA biased so that there is sufficient DC input power to supply for peak RF output power conditions. FIG. 1A further illustrates an exemplary RF input signal 102 to the exemplary PA. As can be seen from FIG. 1A, the PA operates at its highest PAE level at the point where the RF input signal 102 is at a maximum. This is because most of the DC input power to the PA is being transformed into useful RF output power when the RF input signal 102 is at a maximum. As can be further seen from FIG. 1A, the PAE of the PA decreases with decreasing values of the RF input signal 102. This is because at smaller values of the RF input signal 102, less of the DC input power is being transformed into useful RF output power and more is being dissipated by the PA.
To improve the PAE of a linear PA, envelope tracking PAs are often used. The basic idea of an envelope tracking PA is to track the envelope of the RF input signal and use the envelope to modulate the DC input power (or voltage supply) of the PA. As the magnitude of the envelope of the RF input signal decreases, the DC input power of the PA is correspondingly reduced such that the PAE curve of the PA is shifted to the left and the PA remains in a high PAE region. FIG. 1B illustrates a plot 150 of PAE versus RF output power for an exemplary envelope tracking PA. From FIG. 1B, as the envelope of the RF input signal 102 decreases, the DC input power of the PA is correspondingly decreased such that the PA remains in a high PAE region, albeit on a different PAE curve due to the change in DC input power. FIG. 1B further illustrates the increase in efficiency over a non-envelope tracking PA when the RF input signal 102 is at a minimum.
In many devices, multiple envelope tracking PAs are used. For example, in mobile UEs with multiple antennas, each antenna may be driven by a separate envelope tracking PA. Each of the multiple envelope tracking PAs are traditionally implemented independently of each other without any sharing of components between them, which can lead to higher component counts, larger die and/or board area, and increased monetary costs.
The present disclosure will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.